Into heat developable light-sensitive materials, bases or base precursor are frequently incorporated for the purpose of accelerating development by heat. From the viewpoint of preservability of the light-sensitive materials, it is more preferred to employ base precursors which release basic substance upon thermal decomposition. In order to employ such a base precursor in practical use, it is necessary for the precursor to fulfill two somewhat contradictory properties, i.e., stability at normal temperature (e.g., 20.degree. C.), but rapid decomposability at the time of heating.
Basic precursors which have heretofore been known include, for example, a urea as described in U.S. Pat. No. 2,732,299 and Belgian Pat. No. 625,554, a method using urea or urea and an ammonium salt of a weak acid (as described in Japanese Patent Publication No. 1699/65), a method using hexamethylenetetramine or semicarbazide (as described in U.S. Pat. No. 3,157,503), a method using a triazine compound and a carboxylic acid (as described in U.S. Pat. No. 3,493,374), a dicyandiamide derivative (as described in U.S. Pat. No. 3,271,155), an N-sulfonyl urea (as described in U.S. Pat. No. 3,420,665), amine imides (as described in Research Disclosure, No. 15776 (1977)), and a salt of a thermally decomposable acid such as a trichloroacetate (as described in British Pat. No. 998,949), etc.
However, image forming materials using these base precursors have serious defects. Specifically, these compounds cannot fulfill the above described indispensable conditions, i.e., good stability during preservation at normal temperature and rapid decomposition at the time of development processing, and cause problems in that a high image density cannot be obtained, or the S/N (signal/noise) ratio of the image is seriously decreased due to release of the base during preservation.
In order to overcome these defects it has been proposed to use salts of sulfonylacetic acid, as described in Japanese Patent Application (OPI) No. 168441/84 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), and salts of propiolic acid, as described in Japanese Patent Application No. 55700/83. While these base precursors are excellent from the viewpoint of obtaining images of a high density in a short period of time, stability during preservation thereof is still insufficient. In particular, when preserved under conditions of high temperature or high humidity, they cause problems of fog and desensitization.